1. Field of the Invention
The present invention relates to a gas shortage preventive circuit for a fuel cell power generation system. More particularly, the invention relates to a gas shortage preventive circuit for a fuel cell power generation system that is controlled at a constant voltage level, which circuit prevents the occurrence of a shortage of fuel gas in a fuel cell caused by a delayed response of a fuel gas supply system upon an abrupt increase of the load to the system using an overcurrent protective circuit.
2. Description of Related Art
FIG. 1 is a schematic block diagram showing in a simplified manner a conventional fuel cell power generation system having an overcurrent protective circuit. In FIG. 1, a fuel cell stack 1 receives hydrogen-rich fuel gas from a reformer (not shown) and air as an oxidizer gas from an air feed (not shown) and converts chemical energy generated by the electrochemical reaction between hydrogen and oxygen into electrical energy, thus generating power. Output direct current, I.sub.FC, of the fuel cell stack 1 flows to a power transformation circuit such as an inverter 2, which converts the direct current into alternating current. The alternating current is fed to a load 9. The inverter 2 is controlled by a constant voltage control circuit 3. The constant voltage control circuit 3 includes a voltage setting unit 3A which sets up a target voltage, a voltage detector 3B which detects the output voltage V.sub.o of the inverter 2, and a constant voltage controller 3C which performs a proportional and integral action. The constant voltage controller 3C issues a control signal 3S for decreasing any deviation of the detected voltage detected by the voltage detector 3B relative to the target voltage to control the output voltage V.sub.o of the inverter 2 at a constant level. An overcurrent protective circuit 4 includes an overcurrent setting unit 4A, a current detector 4B which detects the output current I.sub.FC of the fuel cell stack 1, a proportional and integral controller 5A, and a voltage blocking diode 5B connected between the constant voltage controller 3C and the proportional and integral controller 5A such that the direction of from the input side of the constant voltage controller 3C toward the output side of the proportional and integral controller 5A is taken as the forward direction. The proportional and integral controller 5A and the diode 5B together serve as an overcurrent controlling circuit 5.
FIG. 2 is a graph illustrating an overcurrent protective area PA for the conventional overcurrent protective circuit. The horizontal axis indicates the output current, I.sub.FC, of the fuel cell stack 1 while the vertical axis indicates a set current value set for the overcurrent setting unit 4A. More particularly, in the conventional overcurrent protective circuit 4, a current value I.sub.100 is set for the overcurrent setting unit 4A. This I.sub.100 corresponds to 100% of the rated output current of the fuel cell stack 1. An area exceeding the value I.sub.100, i.e., the hatched area in FIG. 2, is defined as the protective area PA. If the output current I.sub.FC from the fuel cell stack 1 exceeds the value I.sub.100, the overcurrent protective circuit operates to decrease the overcurrent down to the rated current of the fuel cell stack 1.
Therefore, when the fuel cell power generation system is in a steady state of operation and a prescribed fuel consumption rate (usually 75%) and oxygen consumption rate (usually 75%) are maintained, the overcurrent protective circuit 4 operates as follows. The detected value of the output current I.sub.FC (detected by the current detector 4B) is below the set value I.sub.100, which is set by the overcurrent setting unit 4A. The output from the proportional and integral controller 5A is saturated positively so that the flow of current through the voltage blocking diode 5B is blocked. The voltage setting unit 3A maintains the target voltage level. The inverter 2 is controlled to a constant voltage by the constant voltage controller 3C. On the other hand, when the fuel cell stack experiences an overcurrent reaching the protective area PA, due to a failure such as short circuit on the side of the load 9 connected to the output side of the inverter 2 or short circuit in the inverter 2, the current value detected by the current detector 4B exceeds the set value I.sub.100 of the overcurrent setting unit 4A. When this occurs, the output from the proportional and integral controller 5A is reversed and saturated negatively. This makes the voltage blocking diode 5B become conductive and decreases the target voltage of the voltage controlling unit 3A. As a result, the constant voltage controller 3C issues the signal 3S for restricting the output current I.sub.FC from the inverter 2, thus performing a protective action for lowering the overcurrent of the fuel cell stack 1.
During steady state operation in which the fuel cell power generation system maintains a current value not higher than the rated current, if the load 9 requires a quick increase in power, the inverter 2 (which is controlled to a constant voltage) requires a quick increase in the output current I.sub.FC from the fuel cell stack 1 at a response speed of 2 milliseconds. On the contrary, the fuel cell stack 1 cannot follow such a sudden increase in the power output. It responds in a delayed manner since an increase in the power generated by the fuel cell stack 1 involves an increase in the supply of fuel gas, which is governed by the speed of response of the fuel gas supply system (not shown). The response of the fuel gas supply system is delayed to some extent, so that there occurs a temporary gas shortage in the fuel electrodes in the fuel cell stack 1. In addition, fuel cells have a so-called drooping characteristic, i.e., the output voltage of a fuel cell decreases as its output current increases. Hence, the inverter 2, which is controlled to be at a constant voltage, operates so as to further increase the output current I.sub.FC of the fuel cell stack 1, to maintain the output voltage V.sub.o of the inverter at a constant voltage. This causes the output voltage of the fuel cell to decrease further, thus forming a vicious cycle and finally the fuel cell has a gas shortage. The gas shortage influences to the fuel cell adversely, in that the constituent elements of the fuel cell deteriorate and this causes the power generation and service life of the fuel cell power generation system to decrease.
A fuel cell power generation system with the conventional overcurrent protective circuit 4, whose overcurrent protective area PA is an area above the rated current of the fuel cell, is intended primarily to protect the fuel cell against an excessive current which occurs in the fuel cell stack 1 due to a short circuit on the side of the load or a short circuit in the inverter. Its protective function does not cover protection against overcurrent which will occur due to a sudden increase in the load during steady state operation. Thus, a fuel gas shortage in the fuel cell stack caused by such an overcurrent, and damage to the fuel cell due to the overcurrent, cannot be avoided. In particular, in a light load area where the output current is low, the fuel gas is supplied in a small amount and differs to a great extent from the amount needed to reach the protective level. Hence, a sudden increase in the load in a light load area tends to cause a gas shortage and therefore unrecoverable damage to the fuel cell.